Knowledge of the structure of preproinsulin genes provides the necessary background information and the specific probes to study their expression. By using RNA-blot hybridization and titrations in solutions we are asking the question whether glucose, the physiological stimulus for insulin biosynthesis and release, induces transcription of preproinsulin sequences in isolated pancreatic islets and in insulin-producing cultured cells derived from a transplantable rat insulinoma. To assay the expression of in vitro-modified genes, without depending on the availability of selectable markers, we developed a technique for the introduction of any gene into any cultured cell by microinjection, using iontophoresis. Our results indicate that such transformed cells contain the injected DNA sequences stably integrated into chromosomal DNA. The integration frequency is high. We are currently injecting preproinsulin genes into cellular environments appropriate for their expression. Comparisons of the structure of the preproinsulin gene between different organisms have offered some new insights into the evolutionary process. We are extendng this study to the guinea pig preproinsulin gene(s). We are also studying insulin-like sequences present in human fetal liver and rat neonatal brain.